Default mirror when resizing
If I followed your question correctly: In edit mode, the default scale method is &quot;Individual Elements&quot; which means the scale occurs centered on the selected, connected faces. In the following example, I cut a Suzanne in half and added a mirror modifier:
 You can see her eyeballs popped out and she gets strangely wide. I assume this is what your question is about... To &quot;fix&quot; this, set the 3D cursor to the center of the object (in this example, I left it at the default world origin) and set the scale method to &quot;3D Cursor&quot;. This makes scale behave more like it intuitively should 
I didn't highlight the thing that changed... Look in the top center next to the word &quot;Global&quot;